Relative roles of climate and emissions changes on future tropospheric oxidant concentrations

A Lagrangian chemistry-transport model (STOCHEM) was driven with meteorology derived from a slab ocean general circulation model for conditions appropriate to the present-day and at double CO2, and with emission scenarios appropriate for present day conditions and for the year 2075. The results show conclusively that the effect of including the predicted changes to future climate is to reduce the simulated tropospheric ozone concentrations. The response of global tropospheric ozone in the period 1990–2075 was an increase of 6.4 ppb when both climate and emissions changes were included, compared to an increase of 10.3 ppb when only emissions changes were considered. This difference is mainly due to water vapor and temperature increases, together with some dynamical effects. There are considerable changes to other tropospheric oxidants, with OH, HO2, and H2O2 all increasing considerably in response to climate changes. In contrast, OH decreases when only the emissions are allowed to change. A replicate run of the control scenario with STOCHEM using a different year of meteorology showed considerable interannual variability in local monthly mean ozone concentrations.